Surgical planning for dental implant placement traditionally uses medical imaging modalities to verify the quantity and quality of a jawbone. Multi-planar reformatted CT (computed tomography) has become one of the most comprehensive and accurate aids for implant treatment planning. CT offers means to visualise the oral anatomy of a patient in a number of differently oriented slices of a volumetric scan and to graphically superimpose on the images representations of commercial implants of varying length, diameter and brand (for example, SimPlant™ provided by Materialise, Leuven, Belgium).
According to the current state of the art a patient is sent to a radiologist for a CT scan. The result of the scan is a stack of 2D slices forming a three-dimensional volumetric “data set” which can then be used as input for a dental planning program. Typically, such a program imports the data set provided by a radiology site without altering any information. Using image-processing techniques such as, for example, image segmentation, three-dimensional models of the bone may be derived from the data set. In the planning environment, the practitioner subsequently defines a curve that follows the arch of the jaw. Several cross-sections can be chosen perpendicular to both the panoramic curve and the axial slices. Typically, implant receptor sites may be chosen in these cross-sections. The practitioner can modify the positions and inclinations of each implant as needed in any of the views, including the 3D view. Fine tuning is performed by shifting and tilting of the implant representations or by changing their dimensions. Each individual implant position can be evaluated in terms of the volume of available bone and its quality. The latter is interpretable in CT images by means of the number of Hounsfield units attributed to each of the voxels, i.e. volumetric building blocks of the CT images.
Disadvantages of using CT for dental implant planning are the additional costs related to scanning and the fact that significant irradiation of the patient is required in order to obtain the volumetric data. Moreover CT or CT-like scanning equipment, such as e.g. cone-beam scanners, is costly and has yet to be widely used in the dental market. Scanning equipment is therefore not always readily available.
To overcome these disadvantages methods for simulating dental implant placement without resorting to volumetric image data such as CT, cone-beam scans, MRI scans, have been proposed.
U.S. Pat. No. 6,319,006 describes a method for producing a drill assistance device for a tooth implant in which an X-ray image is correlated with a three-dimensional image of optically measured visible parts of a patient's remaining dentition. The method comprises taking an X-ray picture of the jaw and the compilation of corresponding measured data record, a three-dimensional, optical measurement of the visible surfaces of the jaw and of the teeth and the compilation of a corresponding measured data record. The measured data records from the X-ray picture and the measured data records from the three-dimensional, optical image are correlated with each other. Based on the information that is available, the type and position of the implant relative to the adjacent teeth is planned and a drill template is produced which is attached to the neighbouring teeth, thus making the exact drilling of an implant pilot hole possible.
However, a disadvantage of this way of working is the lack of three-dimensional information about the shape of the jawbone. While the X-ray image provides information about the bone in a projected image, it does not provide insights about the level of the bone in cross section along the dental arch. As an example, FIG. 1 shows two exemplary cross-sections of a jawbone, which on a frontal X-ray image would both be visualized identically. However the bone in both sections is very different as can be seen from FIG. 1.
In WO 2004/064660 a method for dental registration is provided. The method comprises rigidly coupling a base element to a maxillofacial area, inserting an object comprising at least one of a tool and a tool guide into a mouth in the maxillofacial area, and determining a position of the object relative to the rigid element without a reference element outside the mouth. According to embodiments, the method may comprise combining two non-volumetric images of at least part of the maxillofacial area. The non-volumetric images can be obtained by X-ray, optical scan data or direct bone surface measurements. The method may comprise acquiring the first non-volumetric image including at least a part of the base element, identifying at least one registration mark of the rigid element and registering the image to the second non-volumetric image, thereby registering the registration mark to the area.
The method described in WO 2004/064660 is tedious and not so practically convenient because a foreign object, i.e. the base element, needs to be introduced into the mouth and rigidly coupled to the jaw. The object furthermore needs to be present in the mouth during the imaging. Moreover, the technique does not take into account the fact that X-ray images inherently are inaccurate in the lateral direction, as disclosed in the book “Implants and Restorative Dentistry”, Chapter 11—Imaging in oral implantology, page 178. A registration according to the described technique would not compensate for these inaccuracies.